Email from Inbox disappear

Good morning.
We have installed Zimbra in our server as a mail agent. On two occasions, the users have reported that emails from the inbox disappear from the web mail. Upon reviewing the inbox I actually verify that it was empty, and the same happened with the recycle bin. I was wondering if anyone knows of any possible bugs that Zimbra might have in Linux. Otherwise is there a possibility to look what happened in reality? Is there a way to check for example if the entire folder was empty by the user and then the recycle bin?
Thanks and hope someone can help me.

I was wondering if anyone knows of any possible bugs that Zimbra might have in Linux. Otherwise is there a possibility to look what happened in reality?

There are no bugs in any version of Zimbra the removes email, the Trash folder will have older messages removed after a specific period of time (currently 30 days and configurable).

You should check to see if there's any filters that remove email on these accounts. You should also check to see if there's any option set to froward email to another account and not leave a copy on your server. You should also check your daily mail report to see if there any high levels of email being sent by these accounts. If there's any high activity or there's an unknown forwarding address then it's possible the accounts have been hacked - you do enforce strong password security, don't you (if you don't, you should)?